Controlling pressures in gases and liquids requires accurate measurement, and it also means that for monitoring at predeterminable pressure values, switching functions are performed, which trigger, e.g., safety-relevant measures on devices provided for that purpose. A prior-art measuring device of the above-described class is described under the name contact manometer in German Offenlegungsschrift No. DE-OS 23,32,249. Such contact manometers have a mechanical pressure-measuring mechanism, by which the pressure value measured is indicated with a pointer and a dial. To monitor and regulate the pressure, this mechanical pressure indicator is provided with an attachable switching contact, which is actuated depending on the position of the pointer (normally closed contact or normally open contact). On actuation of the switching contact, corresponding information, on the basis of which necessary measures lead to control or even shut-off of the pressurized fluid lines, is sent to a central station via an electric signal line. It is disadvantageous in the mechanically operating display mechanisms with a coupled electrical switching device that the monitoring stations are usually arranged centrally and are therefore at a great distance from the actual measuring point. It is therefore impossible to monitor the pressure lines and to simultaneously measure the actually occurring pressure by means of the prior-art contact manometer. To overcome this shortcoming, the movement of the pointer would have to be converted with complicated technical and electrical means into an analog electrical signal and forwarded. The mechanical system required for this would be complicated, and the accuracy of reading the mechanical display mechanism would be impaired.
A measuring device for determining the pressure of a fluid, which comes closet to the subject of the present invention, is described in German Utility Patent No. DE-GM 76,06,153. An elastic membrane covers a pressure chamber in a pressure-tight manner. On the surface facing away from the pressure chamber, the membrane is provided with strain gauges, which convert the deflection of the membrane generated by the pressure occurring in the pressure chamber into an electrical signal, which is sent to an evaluating unit via (electrical) wires. Even though such pressure sensors make it possible to monitor the pressure signal generated such that a switching function is triggered at a predeterminable pressure value, and this switching function actuates the switching devices that may be necessary, this type of signal processing does have the disadvantage that the electrical pressure signal must continuously be evaluated and monitored via downstream signal processing units, even when the pressure values are far below the predetermined pressure value. The continuous signal monitoring and processing requires unnecessary auxiliary energy for operating amplifier and comparator circuits, which are not needed per se for normal pressure measurement.